1. Field of the Invention
This invention is directed to an interlock device for preventing the operation of a plasma-arc cutting system when necessary parts are not in place. It relates specifically to a plasma-arc torch having a sliding gas valve that interrupts the flow of gas or other working fluid through the torch when essential parts are missing. The interruption of gas flow may be sensed by a flow switch and power to the torch is cut off.
2. Description of the Prior Art
Plasma-arc torches find wide application to tasks such as cutting, welding and spray bonding. These torches operate by directing a plasma consisting of ionized gas particles toward a workpiece.
In the operation of a typical plasma-arc torch, such as illustrated in U.S. Pat. Nos. 4,324,971; 4,170,727; and 3,813,510, assigned to the same assignee as the present invention, a gas to be ionized is supplied to the front end of the torch in front of a negatively-charged electrode. The torch tip, which is adjacent to the end of the electrode at the front end of the torch, has a sufficiently high voltage applied thereto to cause a spark to jump between the electrode and torch tip, thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the torch tip maintains an arc known as the pilot, or non-transferred arc. The ionized gas in the gap appears as a flame and extends externally off the tip where it can be seen by the operator. As the torch head or front end is brought down towards the workpiece, the arc jumps from the electrode to the workpiece since the impedance of the workpiece current path is lower than the impedance of the torch tip current path.
The gas or working fluid is supplied through a conduit from a source of fluid under pressure to the torch tip. Frequently, a secondary flow of fluid is provided which passes through a separate flow path from the first mentioned working fluid for purposes of cooling various torch parts. In this case, the first mentioned fluid is called the primary fluid or gas and the second is called the secondary fluid or gas.
Because the electrode and tip operate in a very high temperature environment, they must be replaced from time to time as they are used up. Accordingly, torches are design to facilitate periodic replacement of these electrodes and tips as well as other parts.
Sometimes, because of operator carelessness perhaps, a tip, electrode or other essential torch part is left off the torch during replacement and not present when the torch is operated. This may cause operator injury. It can also cause damage to the torch. For example, if the tip is not in place the arc generated from the electrode may strike and damage another part of the torch.
The assignee's own U.S. patent application Ser. No. 515,950 filed July 20, 1983, now U.S. Pat. No. 4,585,921 issued Apr. 29, 1986 entitled "Torch Operation Interlock Device," describes an electrical circuit means that functions as an operation interlock when torch parts are not in place. If a sensed part is not in place, the control circuit functions to interrupt operation of the torch, thereby attempting to minimize operator injury and torch damage.
Applicant's assignee is also the owner of two other U.S. patent applications, Ser. Nos. 794,288 and 794,389 filed on Nov. 4, 1985, entitled "Electrode Exposure Interlock Device with Pressure Sensing" and "Electrode Exposure Interlock Device with Flow Sensing," respectively. These devices operate on pressure and flow rate. They use changes in pressure and flow rate of the working fluid to indicate the absence of necessary parts and thereby prevent unwanted operation of the torch.
While a satisfactory solution to the torch parts in place problem, applicant's assignee's prior art devices require more complex construction. The first named device requires a complex electrical circuit. A current path must be established through the parts or parts to be retained. This requires at least one additional wire to form a circuit. Such a circuit thus adds to cost as well as to complexity.
The latter two devices also require fluid or pressure control circuits as well as control systems. They are thus also more complex.